This invention relates to an improved method for making manually disruptable container closures.
More specifically the present invention pertains to providing an improvement in the method disclosed in our U.S. Pat. No. 3,881,437 whereby an integral but fractured section may be more reliably and predictably created in even the alloys of tougher metals, for instance in sheet steel as well as in sheet aluminum.
The referenced method of providing a weakening line in a sheet metal lid, which line is characterized by being a fractured but integral section, contemplates that after a sheet metal closure has, at least in part, been defined by depressing the lid to provide the bounding wall of its closure, a lengthwise indentation by shear-coining will then be made in that formed wall to effect the fractured section. It has since been discovered that when practicing that method on tougher metal alloys, for instance sheet steel instead of aluminum, it can often be difficult to precisely and sufficiently control the tougher sheet material to attain the exact degree of fracture desired or required by means of the subsequent coining operation. Presumably this is largely due to the fact that the metal of the closure wall in the locality to be fractured is, when the prior practice is pursued, under compression and therefore resisting penetration by the coining tool. In softer sheet material this may not be disadvantageous, but in work on sheet steel material, for instance, from which easy-open can tops or the like are to be made, in addition to the more obvious blunting effect upon the coining die, a less consistently uniform degree of fracturing may result with consequently unsatisfactory, because less predictable, strength being imparted to the juncture of the lid with its closure.